Introduction - Lehrstuhl für Thermodynamik

ENERGY CONVERSION IN A HYDROGEN FUELED DIESEL
ENGINE:
OPTIMIZATION OF THE MIXTURE FORMATION AND
COMBUSTION
PETER PRECHTL; FRANK DORER; FRANZ MAYINGER
Lehrstuhl A für Thermodynamik, Technische Universität München,
D-85747 Garching, Germany
C. VOGEL; V. SCHNURBEIN
MAN B&W DIESEL AG, Stadtbachstr. 1, D-86153 Augsburg, Germany
Abstract. A large-bore, four-stroke engine, with high pressure injection and
compression ignition, running with hydrogen, is a modern concept for clean energy
conversion without CO2 emission. The key processes for reliable engine operation are a
good mixture formation, a reliable ignition and efficient combustion. The investigations
of these processes are carried out in a rapid compression machine, with modern optical
measurement techniques.
1. Introduction
The simple reaction of hydrogen and oxygen into water as a clean method for energy
conversion, the high energy density, the wide ignition ranges and the high burning
velocities have been the reasons for a long time for people to investigate the usability of
hydrogen as a fuel for internal combustion engines. Short overviews are given in [1,2].
In recent years, extensive research has been done with different concepts on the
application of hydrogen for engine use. A summary of German activities is given in [3].
The method of internal mixing and compression ignition (C.I.) combines the advantages
of the high efficiency of a diesel engine and the clean combustion of hydrogen and
oxygen. For this combustion method, however, one main problem has to be solved
which is the auto-ignition of hydrogen. A variety of studies on this topic have shown
that a reliable running C.I. engine cannot be realized yet [4,5,6,7]. In a cooperation of
MAN B&W Diesel AG and two departments of the Technische Universtät München, a
large-bore, four-stroke engine running with hydrogen is being developed. The research
project is supported by the Bayerische Forschungsstiftung (Bavarian Research
Foundation). The Lehrstuhl für Verbrennungskraftmaschinen und Kraftfahrzeuge
(LVK, department of internal combustion engines and motor vehicles) is investigating

the behavior of a single cylinder test engine. The Lehrstuhl A für Thermodynamik
(LAT, department of thermodynamics) is investigating the injection and combustion
process with modern optical measurement techniques in a rapid compression machine
(RCM) under realistic conditions. As mentioned above, the ignition of hydrogen under
internal mixing conditions is the key process for a reliable engine operation. Due to this
fact it is main subject of the current research activities at the LAT.
2. Experimental Investigations
In a first phase the experiments are carried out in the RCM by varying of the
compression ratio, the load pressure and the flow condition (variation of the swirl and
the turbulence). The compression ratio reaches a value up to 22, and the compression
end pressure is between 5 and 11 MPa. The temperature reaches values over 950 K. The
hydrogen is injected with a pressure between 25 and 33 MPa, and a maximum injection
duration of 15 ms. In the next step the influence of the injector geometry is analyzed.
For these experiments nozzles with a hole number of 6 up to 24 and a slot nozzle with
nearly equivalent cross-sections were manufactured. These measurements were carried
out under the same pressure conditions.
2.1. EXPERIMENTAL SETUP
The experimental setup used for these investigations is based on an fully optically
accessible rapid compression machine (RCM). The machine simulates a single
compression stroke under realistic conditions. This experimental setup allows a variety
of investigations under different conditions. It is simple to replace the injection device
or the nozzle geometry. The compression ratio, the load, the intake temperature as well
as the injection timing can be varied. In addition to that fact a swirl or a turbulence can
be engendered in the combustion chamber. On the top of the combustion chamber the
electro-hydraulic-controlled injection device for pressurized hydrogen is mounted. The
hydrogen is supplied from single bottles and from bundles. The hydrogen is compressed
with a 30 MPa piston compressor, and it is stored in a small high pressure tank before
the injection. The optical access to the combustion chamber of the RCM is realized
through a large quartz window in the bottom of the piston and additional windows in
the cylinder walls. The RCM allows a maximum combustion pressure over 20 MPa.
This setup allows the use of optical measurement techniques, such as laser-induced
fluorescence (LIF) [8] for the detection of several species like OH, and fast imaging
techniques, such as the high speed digital video and Schlieren techniques to visualize
the mixing and combustion process. A typical setup using the high speed video camera
is shown in figure 1. The dimensions of the machine are in a 1:1 scale to the single
cylinder test engine, which has a piston displacement of about 14 liters and a power
output of approximately 180 kW. The velocity of the piston of the rapid compression
machine corresponds to the piston speed of the engine with 800 – 900 RPM near the top
dead center (TDC). The emitted light (self-fluorescence) from the ignition and the
combustion process is observed using a digital high speed camera at a frame rate of
13,500 Hz. In addition to the conventional data, such as the pressure, the piston

displacement, the needle lift and other required signals, the images are stored digitally.
In the next step the data is analyzed, combined with the images and recorded to a digital
mpeg-film. These films give a good impression of the location and the spatial
distribution of the ignition and combustion process. This method contributes to a better
understanding and an efficient improvement of the combustion concept.
Figure 1: Experimental setup using the rapid compression machine (RCM) and a high
speed video camera to observe the ignition and combustion process
2.2. RESULTS AND DISCUSSION
In the first series of experiments it has been shown that the compression ratio has a
significant influence on the ignition delay. Higher temperatures lead to shorter ignition
delays. However, large variations of the ignition delays have been observed in all the
experiments. The ignition occurs statistically distributed in the combustion chamber.
The analysis of the films proves the assumption that an ignition of a single jet does not
lead to the ignition of other hydrogen jets, as is shown in figure 2. It has also been
observed that several jets have not ignited. It can be inferred that different areas of
combustible hydrogen mixtures of the injected jets in a single compression cycle can
have different ignition delays. As a result, cycle-to-cycle variations in the pressure
recordings are expected, and the possibility of miss-fire can not be excluded. Possible
reasons can be statistical effects on the ignition due to the compression end
temperatures being near the auto-ignition temperature of hydrogen and the varying
mixing conditions. It is likely that there is an influence of the purity of the pressurized
hydrogen, the intake-air or of the leakage in the injection system on the ignition. All

three possibilities were analyzed. The results show that there is no contamination of the
hydrogen compression and of the high pressure storage system. Furthermore, the air
supply system also has no contamination. The injection device has an oil sealing system
which was suspected to leak impurities of oil into the injected hydrogen.
pressure [bar]
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60
40
20
0
260
-0,012 -0,008 -0,004 0,000 0,004 0,008 0,012
time [s]
piston displacement
pressure
needle lift = injection duration
Figure 2: Typical ignition process of a 6 hole nozzle
The installation of another injector with dry sealings has no significant impact on the
statistical ignition behavior. But it has to be taken into account that the hydrogen is used
as it is delivered from a commercial gas supplier. That means it is delivered as a
compressed gas, and it is not as chemically clean as liquid hydrogen (the boiling
temperature of liquid hydrogen is 20 K). The phenomenon of auto-ignition, which is
difficult to realize, and the variations of the ignition delays were also observed and
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280
piston displacement [mm]

discussed by other researchers working on the topic [6, 7, 4, 2]. The ignition delays
estimated from calculations of hydrogen air mixtures with zero dimensional calculations
[9-11] are in a range of 2 ms at compression end temperatures of 1050 K. Siebers [12]
reports ignition delays in his experiments of 0.5 ms at temperatures of 1200 K.
pressure [bar]
100
80
60
40
20
0
pressure
piston stroke
-0,012 -0,008 -0,004 0,000 0,004 0,008 0,012
time [s]
needle lift
Figure 3:Typical ignition process of a slot nozzle with a slot height of 100 um; flame
propagation around the nozzle within 0.4 ms
An enhanced flow field in the combustion chamber, which can be realized
through a swirl or a turbulence, improves the reliability of the ignition and reduces the
variations of the ignition delays. Due to the critical-flow injection of the hydrogen [13]
into the combustion chamber and its high speed in the nozzle outlets of about 1400 m/s
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piston stroke [mm]

(which is the speed of sound), the injected jets are not deflected in the vicinity of the
outlets. Owing to the strong slow-down of the injected hydrogen, a significant influence
on the penetration direction can be observed at a distance of 50 mm from the nozzle.
pressure [bar]
100
80
60
40
20
0
pressure
needle lift
piston displacement
-0,012 -0,008 -0,004 0,000 0,004 0,008 0,012
time [s]
Figure 4: Typical ignition process of a combined nozzle with 6 x 0.6 and 12 x 0.4 mm;
fast flame propagation, fast combustion and good spatial use of the entire combustion
chamber
In order to run the engine under similar conditions like a diesel engine with a
high compression ratio and late gaseous injection, a nozzle geometry with 6 or more
holes is set up. All the experiments with hole numbers up to 10 show that a burning jet
hardly ignites the neighbor jet. As discussed above, this behavior leads to large
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260
piston displacement [mm]

variations in the pressure recordings. As a result of these observations a nozzle system
with a spatially connected combustible mixture distribution was set up. Experiments
with nozzles with 18 holes or a slot show a much better combustion behavior than those
with 6 to 10 holes. Once auto-ignition occurs, the flame propagates very fast over all
injected jets. Due to the equivalent cross-section of the nozzles the diameter of the holes
decreases with the increase of the number of holes. A smaller bore diameter reduces the
momentum of the injected hydrogen significantly (~d 2 ). Owing to a smaller momentum
the jet is slowed down faster and a reduced penetration speed of the jets can be
observed. Due to the smaller speed the auto-ignition begins in the vicinity of the nozzle.
The best ignition behavior is observed with a slot nozzle. Figure 3 illustrates the
ignition process of a nozzle with a slot height of 100 um. Under these conditions an
earlier and a better reproducible ignition can be achieved. On the other hand a longer
combustion duration is observed. The flow conditions have significant influence on the
combustion process. Nozzle geometries with low jet velocities and large ignitable areas
have good auto-ignition properties, but induce insufficient conditions for a fast and
effective overall combustion. The flow energy for the mixing process is lost in highly
throttling nozzles. The nozzles with 6 holes show, in contrast to nozzles with more bore
holes, better combustion properties as soon as the hydrogen is ignited. The combination
of both advantages leads to the application of nozzles with a series of large and small
bores. The existence of the small bores ensure an early and reliable ignition in the
vicinity of the nozzle and a fast flame propagation, which ignites the surrounding jets.
The large bores, however, cause a high momentum of the injected hydrogen, which
leads to good mixing. Figure 4 illustrates the ignition and the combustion process of a
combined 18-hole nozzle with 6 x 0.6 mm and 12 x 0.4 mm bores. A fast flame
propagation around the nozzle is achieved within 0.6 milliseconds. In addition to that
fact a faster penetration of the jets from the large bore holes to the wall of the cylinder is
observed, which results in a more efficient use of the whole combustion chamber.
Nozzles of these types are, at present, the best combination of a reliable ignition and of
an efficient combustion. However, the variations of the ignition processes and the
pressure rise rates are higher than those observed in real diesel engines. In addition to
that fact an improvement of the ignition of the hydrogen near the nozzle can be
expected using slow-opening valves. These types of valves are used in modern
combustion concepts of diesel engines with Common-Rail injection systems, with
which the pressure rise rates can be controlled and the emissions can be reduced.
Although an operation of a C.I. engine using hydrogen seems to be possible
under the above discussed conditions, alternative ignition devices should still be
considered. The low ignition energy of hydrogen suggests the use of a spark plug
ignition under late internal mixing conditions. The varying auto-ignition behavior of the
hydrogen, because of possible impurities, can lead to pre-ignitions under early-injection
conditions. Due to this fact S.I. (spark ignition) hydrogen engines have a reduced
compression ratio of about 8-10 [14 15, 16]. The possibility of setting up a
configuration with late internal mixing and spark ignition, as that used in a new
generation of GDI (gasoline direct injection) engines, is also of great interest. For this
reason experiments have been set up with a small modification to the cylinder head of
the RCM, the insertion of a spark plug. With the same nozzle geometry and a reduced
compression ratio, the auto-ignition can be avoided, which enables a reliable and a fast

ignition of the hydrogen. Due to the optimized arrangement of the holes in the nozzle, a
fast flame propagation around the nozzle is achieved within 1 ms. Figure 5 shows a
spark ignited combustion. The spark-plug timing is set 1 ms after the beginning of the
hydrogen injection. With a late injection near TDC and an immediate ignition with a
spark plug, high compression ratios can be realized, as opposed to external or early
internal mixing.
pressure [bar]
100
80
60
40
20
pressure
0
260
-0,012 -0,008 -0,004 0,000 0,004 0,008 0,012
time [s]
needle lift
piston displacement
Figure 5: Spark ignition of a late injected hydrogen jet
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piston displacement [mm]

3. Conclusions
The developed setup for the investigations of a hydrogen-fueled large-bore C.I. engine
with a high pressure injection system allows a detailed analysis of all relevant
processes. The modern optical techniques enable the analysis of the mixing, the ignition
and the combustion processes. The results obtained with this setup contribute to a better
understanding of the very fast processes. Furthermore this setup allows an efficient
testing and development of engine components. The auto-ignition of hydrogen has been
observed. High pressures and temperatures have a positive influence on a short ignition
delay. Turbulence and swirl support the propagation of the flame over the whole
combustion chamber and regulate the combustion for smoother pressure rates. At
present, combined nozzles containing 18 bores with different diameters seem to be the
best injection geometry to assure small variations of the ignition delay and of the
pressure. However, the current ignition behavior of hydrogen in a C.I. engines without
ignition sources is not applicable yet. Therefore further improvement for a smooth and
safe engine operation is still required. Modern injection concepts with their possibilities
of pilot injection and slow-opening rates offer additional options for improvement.
However, further ignition sources for hydrogen under late internal mixing conditions
have to be considered, and should be investigated.
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